As markets worry about the prospect of a 'hard Brexit' ahead of U.K. Prime Minister Theresa May's speech on Tuesday, a key official in the country's Treasury has called for calm.

"We won't have a hard or soft Brexit, we'll get the possible Brexit," Simon Kirby, economic secretary to the UK Treasury, told CNBC on the sidelines of the Asian Financial Forum in Hong Kong on Monday.

"It's important to note that that the best possible Brexit will not only be good for the UK, but also for Europe and the rest of the world."

A 'hard Brexit' could see the country leave the European Union's (EU) single market and customs union, further complicating immigration and trade with the rest of the EU.